Antibiotic resistance
Also known as: antimicrobial resistance, antibacterial resistance
Antibiotic resistance is the ability of bacteria to survive drugs that would normally kill them or stop their growth. Resistance arises through mutation and the transfer of resistance genes, making infections harder to treat.
Antibiotic resistance is the capacity of bacteria to withstand antibiotics that were previously effective against them. When an antibiotic is used, susceptible bacteria die while resistant ones survive and multiply — a textbook case of natural selection operating on a fast-reproducing population. Overuse and inappropriate use of antibiotics accelerate this selection pressure.
Bacteria resist antibiotics through four main mechanisms: enzymatic inactivation of the drug (beta-lactamases hydrolyzing penicillins), alteration of the drug's target (the modified penicillin-binding protein of MRSA, or the altered peptidoglycan precursors of vancomycin-resistant enterococci), decreased permeability that keeps the drug out of the cell, and efflux pumps that actively expel it. A single organism can combine several mechanisms.
Resistance genes spread in two ways: vertically, as resistant cells divide, and horizontally between bacteria — by conjugation via plasmids (the most clinically important route), transformation (uptake of naked DNA), and transduction (transfer by bacteriophage). Because plasmids often carry multiple resistance genes and can cross species lines, resistance to several drugs can move in a single event, producing multidrug-resistant organisms.
The USMLE Step 1 exam tests specific resistance mechanisms drug by drug and organism by organism, while the MCAT frames resistance within natural selection, prokaryotic genetics, and recombinant DNA topics. Know the four mechanism categories, the three modes of horizontal gene transfer, and marquee examples like MRSA and VRE.
Key takeaways
- Antibiotic resistance lets bacteria survive drugs that once killed them, and antibiotic use itself selects for it.
- The four core mechanisms are drug inactivation, target modification, decreased permeability, and efflux pumps.
- Resistance genes spread horizontally by conjugation, transformation, and transduction — plasmid conjugation matters most clinically.
- MRSA (altered penicillin-binding protein) and VRE (altered peptidoglycan target) are classic exam examples.
- USMLE Step 1 tests mechanisms in detail; the MCAT ties resistance to natural selection and bacterial genetics.